Encryption updates

Credential code in the DscConfiguration module now leverages the [ProtectedData](https://github.com/dlwyatt/ProtectedData) module.  This brings several benefits over the previous version:

- Supports CNG certificates and keys (RSA or ECDH).
- Uses FIPS complaint algorithms and classes.
- Does not place plain-text passwords on disk at any point; SecureStrings are handled in memory.
- Will eventually simplify the DscConfiguration module's code (once support for reading legacy files is removed.)

For now, the module is still able to read files that were encrypted using previous versions.  As soon as you update an existing file's credentials or add a new credential to the file, it will be automatically saved in the new format.  The legacy code may be removed at some point in the future, after a suitable amount of time has passed.